1. Field of the Invention
The present invention relates to a focus detecting apparatus, which attains an improved arrangement of light receiving portions (CCD line-sensors: AF sensors) on a reimaging plane and openings formed on a field mask, which is disposed in front of a reimaging optical system to eliminate unnecessary light. Thus, the openings, hereinafter designated as apertures, are accordingly formed as an area through which only necessary bundles of rays for reimaging are passed to be incident on the light receiving portions.
2. Description of Related Art
A focus detecting apparatus, which is related to the present invention, has been disclosed in the Japanese Laid-Open Patent Publications No. 01-155308 and No. 02-58012. The focus detecting apparatus in these publications works in the following manner: an image seen through an aperture on a field mask is separated by a pair of separator lenses and reimaged on AF sensors so that a focus state is judged by the phase difference of the output of the AF sensors.
On the other hand, the focus detecting apparatus mentioned above has some drawbacks regarding the arrangement of the apertures and the AF sensors. The arrangement of the AF sensors has been the same as that of the apertures on the field mask. For example, if the apertures are arranged in "H" shape on the field mask, the AF sensors are arranged in "H" shape on the reimaging plane. Each arrangement of the apertures and the AF sensors may not cause any functional problems. It should, however, be noted that preferable conditions required for the apertures on the field mask and for the AF sensors are inherently different.
For the apertures, in particular, those formed away from an optical axis of a photographing lens, to minimize an adverse effect of vignetting, it is more preferable for them to be formed in the sagittal direction of the photographing lens, i.e., a longer side of the aperture is placed along a shorter side of a film plane, rather than along a longer side of the film plane.
On the other hand, for the AF sensors serving as the light receiving portions, it is preferable that the AF sensors are disposed, without regard to the arrangement of the apertures, in a manner that an area occupied by the AF sensors is as small as possible for the purpose of space saving in a camera.
If the AF sensors are aligned in a single straight line with a space of minimum necessity, the size of a module in which a reimage forming optical system is contained can be much smaller.
Still further, as position and diameter of an exit pupil of a photographing lens vary due to zooming or due to a type of lens being attached to a camera, a pupil of the focus detecting system may suffer a vignetting; thus, it is preferable that an aperture formed on a peripheral part of the field mask receives a bundle of rays from an area of the exit pupil of the photographing lens closer to the optical axis of the lens.